1. Field of the Invention
The present invention relates to an image display device mounted on a mobile apparatus or the like such as a liquid crystal display device for displaying an image and relates to an image display method for the image display device.
2. Description of the Related Art
When an input image is dark for a display device, which displays an image through modulation of light from a light source, a method for saving power consumption of the light source without changing the brightness of the display image by increasing the brightness of the input image through normalization using the maximum value of a frame in the input image as shown in FIG. 2 and by combining a light source capable of emitting light proportional to the maximum value of the frame (see, for example, JP 01-239589 A).
Since the method is based on normalization using the maximum value of the entire frame abstracted in the maximum value abstractor 12 from the input image, change in quality in the displayed image is quite small. However, when there is at least one pixel with brightness which is close to the maximum, a brighter image cannot be obtained, resulting in a small power saving effect in an ordinary scene. Accordingly the method is used, for example, mainly in a large-screen television receiver required for higher image quality rather than lower power consumption.
A display device for a mobile apparatus is strongly thought to be a simple display for intermediate output in which the number of pixels and the display image size are smaller than those of a final output form such as actual image data, a hard copy, or the like. Accordingly even the image quality is somewhat reduced, a display device with small power consumption has tendency in desirability in view of battery life.
Thus, in recent years, in the case of the display device of mobile apparatus, a method of performing not only the normalization but also gamma conversion for obtaining a brighter image combined by simultaneous reduction of light from the light source to improve the power saving effect has been known as an application of the art. More specifically the darker the average brightness of the entire image is, the brighter the gamma conversion convert the image, adjusting the average brightness value to a value close to a target brightness value. In this case, the power consumption can be effectively reduced since many images can be converted into brighter images by the gamma conversion (see, for example, JP 3430998 B).
Though the power consumption of the light source can be effectively reduced according to the conventional method of performing the gamma conversion based on the average brightness of the input image, there is a problem in that a contrast thereof may be reduced through conversion of a dark input image into a brighter image by the gamma conversion. When the gamma conversion is performed while suppressing the reduction in contrast, there is a problem in that the amount of reduction in power consumption can not be enough.
In many cases, the display device mounted on a mobile apparatus is simply a display for intermediate output in which the number of pixels and the display image size are smaller than those of the final output form. The reduction in power consumption is rather required strongly for the display device on a mobile apparatus. Considering an actual dynamic range of a digital image with around 256 colors, the number of images with no saturation is small since many display images include illumination devices and glossy objects. In view of the circumstances, an optimum amount of saturation for the final output is not necessarily equal to an optimum amount of saturation for the intermediate output.